bioRxiv preprint doi: https://doi.org/10.1101/215814; this version posted November 7, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 1 2 PKD2 is an essential ion channel subunit in the primary cilium 3 of the renal collecting duct epithelium 4 5 6 Xiaowen Liu1,4, Thuy Vien2, Jingjing Duan1,4, Shu-Hsien Sheu1,4, Paul G. DeCaen2,3 and David E. 7 Clapham1,3,4 8 9 1. Howard Hughes Medical Institute, Department of Cardiology, Boston Children's Hospital, 10 320 Longwood Avenue, Boston, MA 02115, USA; Department of Neurobiology, Harvard 11 Medical School, Boston, MA 02115, USA 12 2. Northwestern University, Feinberg School of Medicine, Department of Pharmacology, 320 13 East Superior Street, Chicago, Illinois 60611 14 3. Correspondence should be addressed to P.G.D. or D.E.C. (email: 15 [email protected] or [email protected]) 16 4. Current address: Janelia Research Campus, HHMI, 19700 Helix Drive, Ashburn, VA 20147 17 18 Abbreviations used: 19 - Pkd1 or Pkd2 (lower case letters): when referring to polycystin genes 20 - PKD1 or PKD2 (capital letters): when referring to polycystin proteins 21 - cPkd1: a mouse expressing Pax8rtTA; TetO-cre; Pkd1fl/fl 22 - cPkd2: a mouse expressing Pax8rtTA; TetO-cre; Pkd2fl/fl 23 - Arl13B-EGFPtg: a transgenic mouse expressing multiple copies of the cilia-localized 24 Arl13B-EGFP gene 25 - pIMCD: primary distal collecting duct tubule epithelial cells isolated from mice used 26 in the study (Arl13B-EGFPtg; Arl13B-EGFPtg:cPkd1; Arl13B-EGFPtg:cPkd2) 2+ 27 - [free-Ca ]in: internal free calcium concentration (cilioplasm or cytoplasm) 28 1 bioRxiv preprint doi: https://doi.org/10.1101/215814; this version posted November 7, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 29 ABSTRACT 30 Mutations in either Pkd1 or Pkd2 result in Autosomal Dominant Polycystic Kidney Disease 31 (ADPKD). Although PKD2 is proposed to be an ion channel subunit, recordings of PKD2 ion 32 channels conflict in their properties. Using a new ADPKD mouse model, we observe primary 33 cilia are abnormally long in cells associated with cysts. Using primary cultures of collecting duct 34 epithelial cells, we show that PKD2, but not PKD1, is a required subunit for primary cilia ion 35 channel. The ciliary PKD2 channel conducts potassium and sodium ions, but little calcium. We 36 also demonstrate that PKD2 is not constitutively active in the plasma membrane, but PKD2 37 channels are functional in primary cilia and are sensitized by high cilioplasmic [Ca2+]. We 38 introduce a novel method for measuring PKD2 channels heterologously expressed in primary 39 cilia of HEK-293 cells, which will have utility characterizing Pkd2 variants that cause ADPKD in 40 their native ciliary membrane. 41 42 43 44 45 2 bioRxiv preprint doi: https://doi.org/10.1101/215814; this version posted November 7, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 46 INTRODUCTION 47 Autosomal dominant polycystic kidney disease (ADPKD) is an adult-onset disease characterized 48 by focal cyst development resulting from heterozygous mutations in Pkd1 or Pkd2(Brasier & 49 Henske, 1997; Grantham, 2001; Hughes et al., 1995; Mochizuki et al., 1996). While considered 50 a dominant monogenic disease, the prevailing two-hit model states that ADPKD is recessive at 51 the cellular level and that cysts develop from cells after acquiring a second somatic mutation to 52 deactivate the remaining normal allele(Koptides, Hadjimichael, Koupepidou, Pierides, & 53 Constantinou Deltas, 1999; Pei, 2001; F. Qian et al., 1997; Wu et al., 1998). Mouse models of 54 ADPKD implicate ciliary PKD1 and PKD2 dysfunction in kidney cyst formation. Complete genetic 55 knockout of either Pkd1 or Pkd2 in mice results in embryonic lethality due to structural defects 56 in the cardiovascular system, pancreas, and kidneys(Boulter et al., 2001; K. Kim, Drummond, 57 Ibraghimov-Beskrovnaya, Klinger, & Arnaout, 2000; Lu et al., 1997; Wu & Somlo, 2000; Wu et 58 al., 2002). The onset of kidney cyst development in adult mice following conditional inactivation 59 of Pkd1 or the intraflagellar transport protein kinesin, KIF3a (required for cilia formation), 60 progresses well into adulthood, in analogy to the late progression of ADPKD in 61 humans(Davenport et al., 2007; Piontek, Menezes, Garcia-Gonzalez, Huso, & Germino, 2007; 62 Shibazaki et al., 2008). Kidney-specific conditional repression of either Pkd1 (Pax8rtTA; TetO- 63 cre; Pkd1fl/fl) (Shibazaki et al., 2008)or Pkd2 (Pax8rtTA; TetO-cre; Pkd2fl/fl)(M. Ma, Tian, Igarashi, 64 Pazour, & Somlo, 2013) develop cysts within 10 weeks after the start of doxycycline induction, 65 suggesting that expression of both genes is necessary to prohibit cyst development in mature 66 mice. Recently, the cystic phenotype found in mice deficient in Pkd2 can be dose-dependently 67 rescued by Pkd2 transgene expression(A. Li et al., 2015). Our poor understanding of the 68 functional properties of PKD1, PKD2, and the PKD1/PKD2 complex impedes the development 3 bioRxiv preprint doi: https://doi.org/10.1101/215814; this version posted November 7, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 69 of pharmacological therapeutic strategies - ADPKD is currently treated by dialysis and kidney 70 transplant(LaRiviere, Irazabal, & Torres, 2015). Since PKD2’s function is unclear, it is not 71 currently known if all ADPKD-causing variants in Pkd2 cause a loss of function of the putative 72 ion channel in primary cilia. 73 PKD1 (PC1) is predicted to adopt an 11-transmembrane topology with a large autocleaved (via 74 a G protein-coupled receptor proteolytic site GPS) amino-terminal ectodomain (>3,000 75 residues)(Harris, Ward, Peral, & Hughes, 1995) that is comprised of an array of putative 76 adhesion and ligand-binding modules(Burn et al., 1995; Hughes et al., 1995; F. Qian et al., 77 2002). PKD2 (or PC2, TRPP1, formerly TRPP2) is a member of the large, 6-transmembrane 78 spanning transient receptor potential (TRP) ion channel family(Ramsey, Delling, & Clapham, 79 2006; Venkatachalam & Montell, 2007) and has been observed to form a complex with PKD1. It 80 is proposed to interact with PKD2 through a probable coiled-coil domain(Newby et al., 2002; F. 81 Qian et al., 1997; Tsiokas, Kim, Arnould, Sukhatme, & Walz, 1997), but in other experiments, 82 the PKD1 and PKD2 interaction is preserved in overexpressed systems without the coiled-coil 83 domain and is dependent on the N-terminal domain(Babich et al., 2004; Celic et al., 2012; Feng 84 et al., 2008). Based on biochemistry and immunoreactivity, both proteins can be found in the 85 primary cilium and ER(Ong & Wheatley, 2003; Yoder, Hou, & Guay-Woodford, 2002). In 86 addition, some studies suggest that PKD1 and PKD2 may reciprocally affect each other’s 87 surface membrane or ciliary localization(Harris et al., 1995; Ong & Wheatley, 2003; C. Xu et al., 88 2007). A recent study using inner medullary collecting duct (IMCD) cells derived from human 89 ADPKD cysts suggests that impairing the function of PKD1 or PKD2 negatively affects the 90 localization of the other protein: cells expressing an ADPKD-associated PKD1 mutation that 91 prevents GPS domain cleavage have decreased amounts of both PKD1 and PKD2 in their 4 bioRxiv preprint doi: https://doi.org/10.1101/215814; this version posted November 7, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY 4.0 International license. 92 primary cilia(C. Xu et al., 2007). As discussed below, our understanding of ADPKD pathogenesis 93 is hampered by disagreements about the basic properties of the putative PKD2 current. 94 Furthermore, little is understood regarding what role (if any) the primary cilia have in controlling 95 the progression of cyst formation in ADPKD. Nonetheless, there is no ambiguity in the finding 96 that mutations in Pkd1 or Pkd2 are genetically linked to formation of cysts in kidney and other 97 tissues to cause significant morbidity and mortality in humans(Mochizuki et al., 1996; Ong & 98 Harris, 2015) 99 Previous work reported single channel events in artificial bilayers attributable to an ion channel 100 complex involving PKD1 and PKD2(Delmas et al., 2004; Gonzalez-Perrett et al., 2001; Hanaoka 101 et al., 2000). However, exogenous expression or reconstitution of PKD2 in artificial bilayers have 102 produced contradictory ion selectivity and voltage dependence. The PKD2 ion channel was 103 initially reported to conduct calcium(Gonzalez-Perrett et al., 2001; Hanaoka et al., 2000), and 104 was also blocked by calcium(Cai et al., 2004). Most recently, a gain-of-function mutation 105 (F604P), but not wt PKD2, underlies a measurable current when heterologously expressed in 106 Xenopus oocytes(Arif Pavel et al., 2016). This study demonstrated that plasma membrane PKD2 107 expression does not appear to be hampered by the lack of PKD1, but rather that native PKD2 108 channels appears to be constitutively closed unless mutated (F604P).